Soil Charcoal From Shrublands to Sub-Alpine Forests in the Colorado Front Range.

Soil charcoal C may be sequestered for millennia in temperate forest soils and this is important given that >80% of terrestrial organic carbon (C) is in soils. We measured a super-passive soil C pool, composed of biomass-derived charcoal C derived from wildland fires. Our objective is to quantify soil charcoal C from shrublands upslope to subalpine forests along the Colorado Front Range where wildland fire is and has been an important disturbance component. We hypothesize that differences in wildland fire regimes over the millennia have resulted in baseline soil charcoal amounts. Geospatial data were used to define sample polygons in foothills shrublands (Cercocarpus montanus), and four forest types; ponderosa (Pinus ponderosa), Douglas-fir (Pseudotsuga menziesii), lodgepole pine (Pinus contorta) and spruce-fir (Picea engelmannii – Abies lasiocarpa). These were stratified to an easterly aspect, 10-30% slopes, and a central elevation band within each vegetation type. Soil samples (0-10 cm depth) were analyzed for total soil C and charcoal C via chemical digestion and dry combustion techniques.

Charcoal is most abundant in spruce-fir soils (1.9 +/- 0.92 Mg C/ha) with roughly equivalent amounts in lodgepole pine and ponderosa pine soils (1.4 +/- 1.02 Mg C/ha and 1.4 +/- 0.54 Mg C/ha respectively). The pattern of most soil charcoal at high elevation has been identified in other studies. Charcoal is least abundant in Douglas-fir and shrubland soils (1.0 +/- 0.67 and 0.54 +/- 0.44 Mg C/ha respectively). Overall, soil charcoal is four times more abundant in spruce-fir forests than in foothills shrublands. Spruce-fir forests have most above ground biomass, slow decomposition rates and less frequent wildland fires than the other four types. Shrublands have the least biomass, comparatively rapid decomposition rates and frequent fires. We propose that high biomass and slow turnover rates in the spruce-fir forests creates conditions for net soil charcoal accumulation.